Sputter targets made of ferromagnetic materials are critical to thin film deposition in industries such as data storage and VIS (very large scale integration)/semiconductors. Magnetos cathode sputtering is one means of sputtering magnetic thin films.
The cathode sputtering process involves ion bombardment of a target composed of a ferromagnetic material. The target forms part of a cathode assembly in an evacuated chamber containing an inert gas, such as argon. An electric field is applied between the cathode assembly and an anode in the chamber, and the gas is ionized by collision with electrons ejected from the surface of the cathode, forming a plasma between the target surface and the substrate. The positive gas ions are attracted to the cathode surface, and particles of material dislodged when the ions strike the target then traverse the enclosure and deposit as a thin film onto a substrate or substrates positioned on a support maintained at or near anode potential.
Although the sputtering process can be carried out solely in an electric field, substantially increased deposition rates are possible with magnetos cathode sputtering, in which an arched magnetic field, formed in a closed loop over the surface of the sputter target, is superimposed on the electric field. The arched closed-loop magnetic field traps electrons in an annular region adjacent to the surface of the target, thereby multiplying the collisions between electrons and gas atoms to produce a corresponding increase in the number of ions in that region. The magnetic field is typically created by placing one or more magnets behind the target. This produces a leakage magnetic field on the surface of the target so that the plasma density may be increased.
Erosion of particles from the sputter target surface generally occurs in a relatively narrow ring-shaped region corresponding to the shape of the closed-loop magnetic field. Only the portion of the total target material in this erosion groove, the so-called "race track" region, is consumed before the target must be replaced. The result is that typically only 18-25% of the target material is utilized. Thus, a considerable amount of material, which is generally very expensive, is either wasted or must be recycled. Furthermore, a considerable amount of deposition equipment "down-time" occurs due to the necessity of frequent target replacement.
To solve these disadvantages of the magnetos sputtering process, various possible solutions have been pursued. One potential solution is to increase the thickness of the target. If the target is relatively thick, then sputtering can proceed for a longer period of time before the race track region is consumed. Ferromagnetic materials, however, present a difficulty not encountered with non-ferromagnetic materials. For magnetron sputtering, the magnetic leakage flux (MLF) or leakage magnetic field at the target surface must be high enough to start and sustain the plasma. Under normal sputtering conditions, such as a chamber pressure of 3-10 mTorr, the minimum MLF, also known as pass through flux (PTF), is approximately 150 gauss at the sputtering surface, and preferably is about 200 gauss for high speed sputtering. The cathode magnet strength in part determines the MLF. The higher the magnet strength, the higher the MLF. In the case of ferromagnetic sputter targets, however, the high intrinsic magnetic permeability of the material effectively shields or shunts the magnetic field from the magnets behind the target and hence reduces the MLF on the target surface in proportion to the target thickness.
For air and non-ferromagnetic materials, magnetic permeability is very close to 1.0. Ferromagnetic materials, as referred to herein, are those materials having an intrinsic magnetic permeability greater than 1.0. Magnetic permeability describes the response (magnetization) of a material under a magnetic field. In CGS units, it is defined as: EQU Magnetic Permeability =1 +4.pi.(M/H)
where M is the magnetization and H is the magnetic field. For currently available cobalt sputter targets, the magnetic permeability is approximately 12 or higher.
Because of high magnetic permeability and thus low MLF, and because MLF decreases with increasing target thickness, ferromagnetic sputter targets are generally made much thinner than non-magnetic sputter targets to allow enough magnetic field to be leaked out to the sputtering surface to sustain the sputtering plasma necessary for magnetos sputtering. Non-ferromagnetic targets are typically 0.25 inch thick or greater, whereas ferromagnetic targets are generally less than 0.25 inch thick. Thus, not only can these ferromagnetic targets not simply be made thicker so as to reduce equipment down-time, they must actually be made thinner. To increase thickness, the MLF must somehow be increased.
In the particular case of cobalt sputter targets, the continuing size reduction and increased speed of silicon based integrated circuits has created a need for high purity cobalt targets. Low resistivity contacts with high thermal stability are required for these silicon based integrated circuits, and CoSi.sub.2 is one such contact material. Previously, these contact films were deposited using silicide targets, but these targets, which were made by powder metallurgy processes, had low purity, low density and were frequently non homogeneous. Alternatively, this CoSi.sub.2 film can be grown by vacuum deposition of cobalt onto a silicon substrate, followed by heating to about 500.degree. C. To thereby form the CoSi.sub.2 contact material. For this method, high purity cobalt targets are needed. Furthermore, due to the need for sufficient MLF at the surface of the target for starting and sustaining the plasma, these high purity cobalt targets further require a low magnetic permeability, specifically, lower than the intrinsic permeability of the material.
Several prior developments relate to the reduction of permeability in cobalt-based alloy sputter targets. In U.S. Pat. No. 4,832,810, the permeability is said to be decreased by decreasing the ratio of the f.c.c. (face centered cubic) phase to the h.c.p. (hexagonal close packed) phase. This is purportedly achieved by melting a cobalt-based alloy having a f.c.c. single phase, casting it, and cooling it so as to transform part of the f.c.c. phase into an h.c.p. phase, followed by cold working. The alloy may optionally be hot worked prior to cooling.
In U.S. Pat. No. 5,282,946, the permeability of a platinum-cobalt alloy is said to be decreased by casting the alloy, annealing at 400-700.degree. C., hot working the alloy at a temperature above the recrystallization temperature (i.e., above about 300.degree. C.) to a strain of at least 30%, and cold (or warm) working at a temperature less than the recrystallization temperature.
In U.S. Pat. No. 5,334,267, the permeability of a Co--Ni--Cr--V alloy of f.c.c. structure is said to be decreased by casting the alloy, hot rolling, and cold or warm working to cause work strain to remain in the target.
In U.S. Pat. No. 5,112,468, the permeability of a cobalt alloy with an h.c.p. and cubic structure is said to be decreased by maximizing the h.c.p. Phase and aligning the hexagonal prism axes vertically to the target surface. This is purportedly achieved by casting and forging the alloy, hot rolling at 800.degree. C.-1200.degree. C., followed by quenching and cold (or warm) working below 400.degree. C.
In U.S. Pat. No. 5,468,305, the permeability of a cobalt alloy is said to be decreased by a multiple hot rolling step with a total reduction of 30% or more, followed by cold rolling at a reduction of 10% or less.
In each of these references, the particular alloy or alloys involved presented different parameters that had to be addressed in determining how to lower the permeability of the particular cobalt alloy. Crystal structure, intrinsic permeability, workability, etc. Vary from one alloy to another. None of these references address the particular parameters presented with substantially pure cobalt sputter target materials. For example, the cobalt alloys had f.c.c single phase or multiple phase structures, whereas pure cobalt has a single phase h.c.p. structure at room temperature. Furthermore, as pure cobalt has a lower intrinsic permeability than cobalt alloys, reduction in permeability is more difficult to achieve.
There is thus a need to develop a pure cobalt sputter target and method for manufacturing the same that exhibits low magnetic permeability sufficient for increasing the magnetic leakage flux at the surface of the sputter target.